The primary purpose of food packaging is to protect food against environmental contaminants. Each food category, e.g., frozen, chilled or those which are shelf stable at ambient temperatures, has different requirements. Generally, product processing must be integrated with the package to optimize shelf-life objective. Thus canned foods are both heated and hermetically sealed to exclude microbiological recontamination; dry foods are sealed against moisture access; frozen foods must be maintained at low temperatures and protected against moisture loss; and chilled foods are refrigerated while controlling the transmission of both moisture and air.
A variety of plastic materials is used for packaging depending on the food requirements, e.g., polyethylenes, polyesters, polypropylenes, polystyrenes, nylons, poly vinyl chlorides, etc. Among the structures employed are extrusion and injection blow-molded bottles, thermoformed trays and cast or blown films. High gas barrier plastics are coated, co-extruded, or laminated to structural plastics in order to enhance properties.
The oxidation of fats, oils, and other food components produces off-flavors, off-colors and nutrient loss. Thus, protection against oxygen is one of the most important requirements of food packaging. Common synthetic polyolefin films such as PE and PP are excellent moisture barriers, but they must be coated or laminated with synthetic polymers including EVOH copolymers to provide an oxygen barrier.
EVOH (Ethylene-Vinyl Alcohol) copolymer, is used in co-extruded plastic films to improve oxygen barrier properties. It is, however, a poor water vapor barrier. Even its otherwise excellent OTR (oxygen transmission rate) is sensitive to high humidity. Therefore, for packaging applications, EVOH is usually the core layer of co-extruded plastic films, where it is shielded from moisture by protective layers of polyethylene. Its OTR also depends on its VOH (vinyl alcohol) content.
Sealing is a large concern with high-barrier plastic containers. Plastic cans or bowls are typically double-seamed with a metal end. The seam is critical to the safety of the product.
There are many kinds of plastic containers that are made with different technologies. One family of containers is made of thin sheet formed into a tube shape. Such containers may be made from paper, cardboard, PVC, PET, acrylic, plastic and polythene. The containers can have all sorts of ends fittings, such as beaded, curled, metal seamed ends, ring pull closures, plugs, caps and many others.